Currently, intensive research is being made into organic nonlinear optical material showing a great nonlinear optical characteristic as a second order nonlinear optical material. For example, representative molecular crystalline materials include MNA(2-methyl-4-nitroaniline), m-NA (m-nitroaniline), MAP (methyl-(2,4-dinitrophenyl)-aminopropanate), POM (3-methyl-4-nitropyridine-1-oxide) (for example, M. Kato, al., et eds.: "Organic nonlinear optical materials," CMC, (1985); D. S. Chemula et al.: "Nonlinear optical properties of organic molecules and crystals," vol 1, 2, Academic Press, (1987); etc.), chalcone (Japanese Laid open Patent Hei. 3-112982), and many other compounds.
Known examples of organic ionic crystals include, L-arginine phosphate monohydrate (LAP) of salt of L-arginine and phosphoric acid, salt of 2-amino-5-nitropyridine and phosphoric acid (2A5NPDP) (K. Kotler et al.: J. Opt. Soc. Am. B Vol. 9, (1992), page 534), salt of 2-amino-5-nitropyridine and tartaric acid (ANPT), and so on (Watanabe, et al., abstracts from the 62nd fall meeting of the Japan Society of Chemistry, (1991), 2I08).
Moreover, a laser device containing a wavelength converter using molecular crystal as the organic nonlinear optical material (S. Ducharme et al.: Appl. Phys. Lett. Vol. 57, (1990), 537, Y. Kitaoka et al.: Technical Digest of the Conference on Lasers and Electro-Optics (CLEO), 1991, CFA8, etc.), a laser device containing an optical waveguide-type wavelength converter (A. Harada et al.: extended abstracts of the 39th Spring meeting of the Japan Society of Applied Physics and Related Societies, (1992), 29pZR-2), and others are known.
The laser device utilizing the wavelength converter composed of organic nonlinear optical material has the following problems.
When using molecular crystal as the organic nonlinear optical material, since molecules are bonded with each other by weak Van der Waals bonds and/or hydrogen bonds, it is often difficult to grow large crystals. If a crystal is grown of sufficient size, since the crystal is soft, it is difficult to obtain a optically good polished surface. Thus, the scattering components at the crystal surface increase and the transmittance of fundamental waves is lowered thereby providing an insufficient wavelength converter function.
In the light of such molecular crystal problems, organic ionic crystal, which introduces ionicity of large bond strength in organic molecules, has recently come to be considered. In organic ionic crystal, since stronger ionic bonds than Van der Waals bonds or hydrogen bonds are formed in the crystal, a single crystal of relatively large size and great mechanical strength can be easily obtained. However, in the LAP, for example, since the nonlinear optical constant is small, the conversion efficiency of laser light of low power is small, and is not suited to wavelength conversion of semiconductor laser light. Furthermore, for example, in 2A5NPDP or ANPT, although the nonlinear optical constant is higher than that in LAP, if light absorption of a fundamental wave of near infrared to infrared light in a wavelength band of about 1 .mu.m is present when used in a wavelength converter, a sufficient wavelength conversion efficiency is not obtained.
When using organic nonlinear optical material of molecular crystal as the wavelength converter of a laser device, a favorable crystal end surfaces are not obtained because the crystal is soft and feeble. As a result, the scattering component at the crystal surface increases, and good transmittance of a fundamental wave cannot be obtained, and a laser device provided with a wavelength converter of high efficiency cannot be obtained. If, for example, favorable crystal surfaces are obtained, a temperature rise occurs partially due to the fundamental wave light concentrated in one part of the crystal, and the refractive indeces to fundamental wave and higher harmonics are altered. The phase matching condition (the condition for propagation of fundamental wave and propagation of higher harmonics at the same phase velocity in the crystal) is thus broken, and therefore the conversion efficiency may not be enhanced if the strength of the fundamental wave is increased. To solve this problem, it has been proposed to adhere sapphire plates of high thermal conductivity to the crystal end surface (Sasaki et al.: Japanese Journal of Optics, Vol. 21, No. 5, 1992, page 284) to remove the heat accumulated in the crystal. However, heat release has its own limit because the heat generated inside the crystal is released through the sapphire plates only after the heat reaches the crystal surfaces by propagation through the molecular crystal, with its poor thermal conductivity. In addition, the sapphire plates are expensive.
Using an organic ionic crystal, since organic molecules are mutually bonded by strong ionic bonds, crystal growth is generally easy, and crystal hardness is expected to be high. In the laser device using a wavelength converter of ordinary organic ionic crystals, light absorption occurs in the near infrared to infrared region of the wavelength 1 .mu.m band. And a large fundamental wave strength is not obtained in an opical resonator. Therefore a laser device with an efficient wavelength converter cannot be obtained.